Hydraulic fracturing is the well stimulation technique designed to increase the productivity of a well, such as an oil well, by creating highly conductive fractures or channels in the producing geologic formation around the well. The process involves injecting a fluid at a high rate and high pressure to rupture the formation and create cracks in the rock and pumping into these cracks a fluid containing a particulate material (propping agent or proppant) to maintain the cracks or fractures open by resisting the forces which tend to close the fractures. Thus, the function of the propping agent is to provide high permeability in the propped fracture. Hydraulic fracturing has been used with increasing frequency to improve the productivity of gas and oil wells in low permeability reservoirs.
The list of materials used in proppants is rather long and includes: sand (the most common proppant), nut shells, aluminum and aluminum alloys, wood chips, crushed coke, granulated slag, pulverized coal, crushed rock, granules of metal such as steel, sintered bauxite, sintered alumina, refractories such as mullite and glass beads. Although sand is still the most prevalent proppant, at closure stresses encountered in deep formations, it tends to disintegrate, producing fines or fragments which can reduce permeability of the propped fracture. This tendency begins at closure stresses above about 5,000 psi (34.5 MPa).
The proppants recently developed to withstand increased overburden pressure in deeper wells are sintered bauxite (see e.g. U.S. Pat. No. 4,068,718) and zirconium oxide (see U.S. Pat. No. 4,072,193). Coated proppants have also been proposed in the patent literature (see U.S. Pat. No. 3,376,930 on metal coated proppants and U.S. Pat. No. 3,026,938 on plastic coated proppants).
Sintered bauxite has high sphericity and good chemical stability in well formations. However, its cost is much greater than that of the more common sand proppants. Since the specific gravity of bauxite is substantially greater than sand, the cost per unit volume of bauxite is even more expensive than sand.
The higher specific gravity of bauxite affects the transport of the proppant into the fracture. Generally speaking, lower specific gravity proppants can be carried further into fractures than those of higher specific gravity. Lower specific gravity proppants permit decreases in pumping rates during proppant placement which in turn reduces bottom hole pressure. The reduction of bottom hole pressure is felt to limit the vertical propagation of fractures (horizontal propagation being desired). In addition, lower proppant density allows for the use of less expensive fracturing fluids.
According to a study done for the U.S. Department of Energy, published in April 1982 (Cutler, R. A. and Jones, A. H., "Lightweight Proppants for Deep Gas Well Stimulation" DOE/BC/10038-22) an ideal proppant for hydraulic fracturing would have a specific gravity less than two, be able to withstand a closure stress of 138 MPa;, be chemically inert in brine at temperatures to 200.degree. C., have perfect sphericity (a Krumbein roundness of 1), cost the same as sand on a volume basis, and have a narrow proppant size distribution. The same report admits that such a proppant is not likely to be forthcoming in the foreseeable future; however, it states that a proppant capable of withstanding closure stresses of 85 MPa, having a specific gravity of 2.6 to 3 and costing 1/3 to 1/2 the price of bauxite would solve about 90% of the current hydraulic fracturing problems.